Pondering

[Moosetracks]

I have been interested in solar water heating for a long time. I purchased a couple of vacuum tubes after seeing YouTube videos of boiling water in the freezing cold. I am an engineer by degree and manufacturing equipment designer and seeing this tube boiling water like this has always let me wondering why it isn’t utilized more.
I have scoured the internet and seen how many people use these systems to create hot water and some supplemental heat. One of my favorites was of a guy who buried a septic tank insulated it and used it to store hot water for his DHW and some heat.
From reading this forum I see there is a lot of solar experience and knowledge and I was hoping you guys would be able to help me understand the dynamics of this stuff a little better.
First of all, why aren’t there people using this system to create electricity by harnessing the heat to create steam? I see google doing it with a massive array of mirrors aiming the sun at a pressure vessel. Couldn’t you put enough of these tubes in a system to generate enough steam to power a small generator? Are PV cells more efficient at creating electricity than using heat?
What is a realistic maximum temperature to get out a solar vacuum tube collector or flat plate collector? I see lots of people store heated water, but why not use sand and a heat transfer fluid to store at higher temperatures? That way you would have more energy to use at night. It also seems like that would be easier than trying to build these wooden water tanks I read about people using. Sand seems like it would be much easier to deal with than a 500 gallon tank of water.

[DanS26]

.......... I see lots of people store heated water, but why not use sand and a heat transfer fluid to store at higher temperatures? That way you would have more energy to use at night. It also seems like that would be easier than trying to build these wooden water tanks I read about people using. Sand seems like it would be much easier to deal with than a 500 gallon tank of water.

Yes, lots of people use heated water to store excess energy. I use (in the colder months) ceramic bricks....pretty close to the heated sand idea you have. It's important to know the system dynamics....energy costs, heat loss, infastructure needs, maintenance, building codes, etc.

A brief discussion is here:

https://www.solarpaneltalk.com/forum/solar/the-pros-and-cons-of-solar-energy/304198-would-an-ets-electric-thermal-storage-system-work-for-you

[J.P.M.]

From one engineer to another: I've spent a career around power and process engineering as a professional enginer and around solar energy as more than a hobby since about 1975 or so. At this time, small residential scale power generation is usually more cost effective and practical if done with PV.

If you are seriously interested, see the engineering literature for discussions of the history, methods and limitations of using the sun to generate power, or for process heating applications.

Or, start with a book titled: "A Golden Thread", by Ken Butti and John Perlin, ISBN # 0-442-240058-8. You will see there's not much new under the sun and much more than can be covered in a forum format.

[Moosetracks]

Dan,
I read your other post and from what I can tell your using the ceramic bricks as a thermal mass in the room with your electric heater to heat them up or do you heat the bricks hotter and then dissipate that heat over time? Do you have more details about the specific device or setup you have?


JPM,
I figured since its not done there was a reason. In your opinion whats keeping it from competing with solar cells even on larger than residential scales? Is it that PV is more set it and forget it than using steam, and large scale production isn't feasible because it can only supply power under daylight somewhat clear weather conditions? Thanks for the info I'll check that book out.


Do either of you have solar vacuum tubes? I am curious about the realistic maximum temperatures you can heat an external mass to. I have read about stagnation temperatures, but I am assuming these devices have a non linear btu output depending on what the temperature of the external mass they are heating although I haven't seen any performance curves that show this effect.

[Amy@altE]

I have quite a bit of experience with solar space heating. I designed and built a system for my house in Massachusetts using flat plate collectors, but the results would have been the same with evacuated tubes. The thing to keep in mind is that most of the solar thermal loops are pressurized, thus raising the boiling and steam temperatures. When the water is circulating, the max temperature you'll generally see is 180F, and that's too low for steam.

There is only so much heat the collectors can pull from the sun in the winter when the heating is needed. To get the number of collectors needed to make it economically viable is very difficult. Sizing the system big enough to gather enough heat results in a very large array. That's a very large bill. I used four 4'x8' flatplate collectors to provide about half of the heat for my 760 sqft home. It worked quite well, and was affordable because I got used collectors for free, and discounts on the balance of system. I was also offsetting electric heat, which is insanely expensive here in New England. You can see on the specs of the collectors how many BTUs it is able to collect and under what conditions. You can then calculate how many would be needed to meet your heating needs.

Bob Ramlow has a great book that describe heating your house with solar thermal and a sand bed. It's called Solar Water Heating - Revised and Expanded. It works great for his house in Wisconsin. I've been there in the winter and his house was nice and warm. But he had a very large array of thermal collectors to make that heat.

These days the price of solar electric has come down to the point where it is economically better to do solar electric, except in some cases, mostly commercial. But when you are trying to make heat in the heating season with solar thermal, the math is against you.

Amy
(formerly the Solar Thermal Queen)

[LucMan]

A few years ago I posted a simulation for solar space heating on this site. Do a search for T sol annual simulation. This graph will show you why solar space heating doesn't work. Or that it could possibly work with an extremely large array to meet the degree day demand if you had unlimited funds to throw at the system.

[DanS26]

Dan,
I read your other post and from what I can tell your using the ceramic bricks as a thermal mass in the room with your electric heater to heat them up or do you heat the bricks hotter and then dissipate that heat over time? Do you have more details about the specific device or setup you have?...........

Moose, I purposefully did not include references to ETS product manufacturers so as not to violate this site's advertising rules.....but since you asked I'll reference the two major ETS manufacturers in North America:

http://www.steffes.com/

http://www.dimplex.com/en/renewable_energy/storage_heaters

I use the Steffes products. Their product line runs from a small 120v plug in room heater to all the way up to furnace sized whole house systems. I use the medium sized room systems.

The ceramic bricks in these systems are heavy and will store a lot of heat energy. They are very well insulated but still will lose a certain amount of heat just resting as with any thermal system. Since I have two units now, I charge one in the morning from 10am to 1pm and charge the second from 1pm to 4 pm. The bricks get charged based on parameters set and using an outside thermostat to determine a charge rate. I also have programmed in nighttime temperature setbacks. A vacation switch will shut the system down for absences or if I anticipate a period of dismal weather. My goal is to only use my PV system to charge the thermal mass.

These systems are not for everyone. You need low cost PV power that will otherwise be lost or only able to sell at wholesale rates to be economically feasible. Resistance heat is expensive if you are using retail rate electricity.

[J.P.M.]

JPM, I figured since its not done there was a reason. In your opinion whats keeping it from competing with solar cells even on larger than residential scales? Is it that PV is more set it and forget it than using steam, and large scale production isn't feasible because it can only supply power under daylight somewhat clear weather conditions? Thanks for the info I'll check that book out.

Do either of you have solar vacuum tubes? I am curious about the realistic maximum temperatures you can heat an external mass to. I have read about stagnation temperatures, but I am assuming these devices have a non linear btu output depending on what the temperature of the external mass they are heating although I haven't seen any performance curves that show this effect.

I do not have vac. tubes as I have no need for process heat. As for reasons why it's not cost effective: Too complicated for the average small system and homeowner and probably less reliable.

As an engineer you already know that it is best to match the source to the task. One big reason why electric resistance heat is so expensive is because electricity is a very versatile (low entropy) energy source. Using all that versatility to simply generate heat is a waste, not unlike using a gold bar for a paper weight when lead is a s fit for purpose. Using the PV electricity to power a heat pump is a slightly different story, and may have some merit from a process costing standpoint.

Using vac. tubes to generate higher temps. than needed for the task of domestic space heating is more wasteful than matching a lower temp. source of energy, say, at 50-60 deg. C. fluid from a flat plate thermal collector for example, to a low temp. need like space heating.

Vac. tube thermal devices are able to generate higher temperatures than flat plate collectors. However, do not confuse quality of heat (temperature) with quantity of heat ( BTU's, or calories).

Vac. tube solar is great if you need high temps for process heat applications or steam, but for domestic space heating, vac. tubes will usually produce less quantity of heat for more hassle and service requirements, and are usually more wasteful than using lower temp. sources.

The entropy matching of source to need is one reason why, in some climates that are sunny, passive solar for domestic space heating can be very cost effective as a heating assist method if done properly.

[LucMan]

Here is the link. The proposed system consisted of 8- Bosch sks 4.0 hybrid thermal panels and 200 gallon storage for a 1500 sq ft area with radiant floor and low temperature radiators.

https://www.solarpaneltalk.com/filedata/fetch?id=279195